1. Field of the invention
Aspects of this invention relate generally to bucking systems and to methods of substantially cancelling a magnetic field at points within a volume. More particularly, aspects of this invention may be used in electromagnetic prospecting to cancel the effect of a large transmitted field on a magnetic field sensor without appreciably modifying the interaction of the transmitted field with the ground. The current invention facilitates such cancellation when the sensor is displaced relative to the transmitter.
2. Description of the Related Art
Electromagnetic (“EM”) exploration methods comprise an important part of the geophysical methods used to map the Earth in the search of oil, gas and mineral deposits, aquifers and other geological features. EM methods can broadly categorized into two categories, passive source methods in which an electromagnetic survey apparatus is used to map naturally occurring time-variations of the electromagnetic fields over the surface of the Earth, and active source methods, in which the electromagnetic field is emitted from a transmitter that is an integral part of the survey apparatus.
Active source EM systems comprise several parts; a transmitter and antenna to create an electromagnetic field, a sensor and a receiver to detect the signal from the transmitter, and related electronics, mechanical elements, data recorder and a power source. Although EM systems also comprise passive systems in which the natural variation of the electromagnetic field is measured absent a transmitter, in the following discussion, EM systems shall be understood to comprise only those systems with a transmitter unless otherwise noted.
Active source EM systems operate by supplying a time varying current waveform to a transmitter coil, or loop, which creates a corresponding “primary” time varying magnetic field. Time variations in the primary field then induce eddy currents in the Earth, resulting in “scattered” magnetic fields. The scattered fields, together with the primary field, are measured with a receiver, usually by employing a coil, loop or magnetometer sensor. Characteristics of the scattered magnetic field may then be used to determine the electrical properties of the ground. These properties may then be used as a basis for geological interpretation such as inferring the presence of geological features. For example, the characteristic of the scattered field that is in-phase with the primary field may be of interest for detecting highly conductive ores. Improving the characterization of the scattered magnetic field leads to improved geological inferences and hence to the success of any prospecting venture employing an active source system.
In the following, “coil” and “loop” may be used to mean the antenna through which the primary field is emitted, and either may comprise one or more windings (turns) of electrical conductor. The resulting magnetic fields are then detected with a receiver that includes one or more magnetic field sensors. A magnetic field sensor may be a coil, loop or circuit element in which changes in the magnetic flux density are detected in accordance with Faraday's Law, or it may be a magnetometer. Examples of magnetometers include devices that employ fluxgate, feed-back coil, Hall effect, and optically pumped atomic vapor principles for detecting the magnetic field, as well as related instruments.
Loops and coils may comprise circular, elliptical, oval, helical or other similar rounded shapes, or sections thereof, and may comprise linear segments which together form a closed shape, usually with internal angles of less than 180 degrees, examples of which are rectangles, hexagons, octagons, dodecagons and so forth. Loops comprise at least one conductive winding, generally composed of an electrically conductive substance such as copper or aluminum, but may comprise a superconductor. Loops fashioned as convex symmetric polygonal shapes with a plurality of sides may be considered to be substantially circular, as would a circular loop.
When an EM system is deployed in the air, one of two configurations are usually employed. In the first configuration, the transmitter and the receiver may be located on the same platform, structure or “carrier”, while in the second configuration, the receiver may be towed at some distance behind the transmitter. In the first configuration, the transmitter and the receiver may be mounted on an aircraft “carrier”, examples of which include the system once operated by the Geological Survey of Finland and the Hawk system built by Geotech Ltd. It is also possible to mount the transmitter and receiver on a platform or chassis “carrier” which is towed from the aircraft. Such carriers are generally towed beneath helicopters, and are often referred to as “birds”, “sondes” or “bombs”. In such cases, the bird may be typically towed 30 to 60 meters below the helicopter at altitudes of about 30 to 60 meters above the ground. In such systems, because the transmitter and receiver are located in close proximity, the primary field at the receiver may be orders of magnitude larger than the scattered field.
When the primary field is much larger than the scattered field, a means of primary-scattered field separation is required to permit accurate detection of the much smaller scattered field. One common method of accomplishing this is by time separation, whereby the primary field is broadcast as a series of shaped pulses with alternating polarity, with each pulse separated by an off-time during which no current flows in the transmitter loop. If the scattered fields are measured during this off-time, the primary field will not be present and highly sensitive measurements of the scattered field are possible. The disadvantage of limiting measurement to the off-time is a loss of information. In particular, the in-phase component of scattered response may be poorly rendered, with the result that certain highly conductive ores may be undetectable. Since highly conductive ores are often targeted in airborne electromagnetic (“AEM”) surveys, accurate on-time measurements may be quite important to the success of AEM ventures. It is therefore advantageous to acquire good quality in-phase AEM data.
A number of AEM systems have used off-time measurements as a means of separating the scattered field from the primary field. Most notable of these was the Barringer Input system and the systems derived from it such as Geotem, Megatem and Questem.
Bucking provides an alternative means of primary-scattered field separation. When the in-phase component of the primary field is large, such as when the transmitter and the receiver are located in close proximity, a bucking loop may be used either to directly cancel the primary field at the receiver through active bucking, or to cancel its effect on the receiver through passive bucking. Active bucking involves the creation of a bucking magnetic field that will substantially cancel the primary field seen by the magnetic field sensor of the EM system. Usually the bucking magnetic field is created by passing the time varying current waveform used to energize the transmitter loop or antenna through a second smaller loop that is near the magnetic field sensor. In passive bucking, an additional magnetic field sensor is used to detect a different combination of primary and scattered fields than is seen by a single magnetic field sensor. The signals from the two sensors are then combined in a way to annul the primary field in the combined signal. Bucking may therefore be used advantageously to acquire good quality in-phase AEM data in the presence of a large primary field.
An additional advantage to bucking results as a consequence of suppressing the primary field in the presence of the receiver. When the primary field is bucked, the receiver may be operated with higher sensitivity than were the field to be unbucked. More subtle scattered field anomalies may therefore be detected, so permitting the detection of smaller geological features with weaker physical property contrasts without saturating the receiver.
Examples of systems using bucking are the Dighem helicopter frequency domain system which employs passive bucking, as did a proposed system by Whitton (US patent application 2003169045A1); and the VTEM (US patent application 2011/0148421 A1) and the Aerotem helicopter time domain systems that employ active bucking.
In systems employing active bucking, the objective is to annul the primary field at the receiver without appreciably affecting the eddy current induction caused by the transmitter in the Earth. Accordingly, the bucking loop is chosen to be geometrically smaller than the transmitter loop but closer to the sensor. As a result, the range of receiver positions over which the field may be bucked is usually also small. Because of this, any relative displacement of the magnetic field sensor with respect to those loops may strongly affect the degree to which the primary field is cancelled at the sensor. Accordingly, in the current state of the art, the quality of the bucking improves as the system becomes increasingly rigid.
An advantage of active bucking is that the primary field in the vicinity of the receiver is suppressed, despite the fact that the field is not perfectly cancelled at all nearby locations. In so doing, eddy current induction due to changes in the primary field within any metallic components of the receiver and its chassis is strongly reduced.
In the current state of the art, bucking has been most effective when the relative geometries of the transmitter loop, the magnetic field sensor and the bucking loop are nearly rigidly fixed. Whenever the loop geometries change either in shape or in position relative to one another, unbucked residuals of the primary field will appear as signals in the receiver. The residuals are generally indistinguishable from the in-phase scattered field, and so may degrade the quality of the measured scattered response. The AEROTEM and Dighem systems employ a nearly rigid geometry, and so minimize the variation in unbucked primary field residuals caused by loop motion. Nevertheless, some unbucked residuals may occur, even in a system with a nominally nearly rigid geometry. These residuals may result from small changes in loop geometry, often attributed to thermal expansion, producing a phenomenon known as “drift”.
Despite the advantages of a rigid geometry for accurate bucking, and so for measuring the in-phase component of the scattered field accurately, it may be necessary or advantageous to permit some variation in the relative geometry of the transmitter loop, the magnetic field sensor, and the bucking loop. The VTEM system is illustrative of an AEM system that is substantially rigid in the EM acquisition band, yet has a flexible geometry. The light weight of its transmitter chassis permits a larger transmitter loop and therefore moment than would be possible were the system to be nearly rigid. Because the transmitter loop is deformable, it can be handled with greater ease during lift-off and set-down stages of each flight. Construction of the loop chassis in sections facilitates transportation and breakage is easier to repair: Collisions do not involve the catastrophic loss of a single rigid chassis with its high-value components. The trade-off introduced as a result of increased flexibility is that the fidelity of the bucking is less than could be provided by a comparable nearly rigid system.
Polzer et al (international patent application WO 2011/085462 A1) has noted a second advantage to allowing some flexibility in the geometry of the transmitter loop, the magnetic field sensor and the bucking loop . Polzer noted that the rotation of an EM sensor in the background magnetic field of the Earth, particularly in the 1-25 Hz low frequency range, creates noise which had previously prevented the acquisition of high-precision airborne electromagnetic data in that band. By employing a stabilization system for motion isolation in which the magnetic field sensor moves relative to the bird in which is housed, high-precision airborne electromagnetic data in the 1-25 Hz band may be acquired. In so doing, the geometry of the AEM system must be flexible.
Thus, in the current state of the art in AEM surveying, single loops are used to buck the primary field. Nearly rigid systems provide relatively stable bucking and permit precise in-phase measurements of the scattered field, sacrificing transmitter moment, light weight and certain logistical advantages. Flexible systems permit a larger transmitter moment and logistical advantages, but with less perfect bucking, and less accurate measurement of the in-phase component of the scattered field as a consequence. Less accurate in-phase measurements may result in poorer resolution of highly conductive geological features, many of which are targets of EM surveys commissioned for mining exploration. Less perfect bucking could also mean that larger magnetic field amplitude variations may be encountered than in the case of a well bucked system, and that as a consequence, EM data may be acquired with lower resolution.
Bucking coils are not necessarily used with the intention of annulling the field of a transmitter. For example, US Patent application 2011227578 A1 to Hall et al describes an induction logging tool which uses multiple bucking coils to redirect the field produced by the transmitter at any angle from the rotational axis of the logging tool.
Miles et al, in U.S. Pat. No. 7,646,201 B2, disclosed an AEM system having a rigid transmitter loop concentric with an inner and an outer receiver loop. By null coupling the receiver loop to the transmitter, the receiver loop could be made mainly sensitive to scattered field of the Earth generated within the annulus defined by the receiver loop.
Kuzmin et at (Patent application US 2010/0052685) disclose an active bucking system for the VTEM AEM system which has a flexible geometry. The system consists of an outer transmitter loop and an inner, coplanar and concentric bucking loop, both of which are centered on a receiver loop. The bucking loop and the transmitter loop are connected in series so that the primary field at the receiver is approximately annulled. However, flexure in the loop geometry causes shifts in the measured fields resulting from unbucked residuals of the primary field at the sensor. In the case of systems such as Kuzmin's where the transmitter and bucking loops are approximately concentric around the magnetic field sensor, the axial magnetic field, Hz, though the center of each loop may be computed, to good approximation, from:Hz(z)=i/{2*a*(1+(z/a)2)3/2)
where i is the current in the loop, a is the radius of the loop and z is the offset on the axis through the loop.
It would be advantageous, in the case of AEM systems, if the bucking apparatus could be designed so as to accommodate relative motions of the transmitter loop, magnetic field sensor and bucking loops so as to retain the advantages of system flexibility as is in the case of the VTEM system while improving the bucking within a volume defined by the motion of the magnetic field sensor relative to the transmitter and bucking loops. Such a bucking apparatus would be advantageous in flexible EM systems, and would improve bucking in AEM systems employing motion isolation, as in the case of Polzer's system. A first advantage of such a bucking apparatus would be in yielding improved in-phase EM data, and so improved sensitivity to highly conductive ores . A second advantage would be in yielding data which may be acquired with improved resolution, resulting in greater sensitivity to subtle features in the scattered electromagnetic field.